Increasing bandwidth demand has been driving communication systems to higher capacities. Therefore, there is a strong motivation to enhance spectral efficiency to increase the total capacity. Employing optical orthogonal frequency division multiplexing (O-OFDM) modulation to transmit signals can realize high-spectral efficiency and long distance transmission. To achieve high receiver sensitivity with coherent detection based on digital signal processing, the bandwidth of the analog to digital converter (ADC) and the sample rate may be high. Usually, the ADC bandwidth may have two times of the bit rate of the signal, and the sampling rate may be four times of the bit rate. For example, if each subcarrier of the OFDM signal is 25 Gbaud Quadrature Phase Shift Keyed (QPSK), the ADC bandwidth should be 50 GHz and the sample rate should be 100 GSa/s to obtain optimum results. However, an ADC with these specifications may not be commercially available. Therefore it would be advantageous to reduce the ADC bandwidth and sample rate while maintaining the same performance.